A split pin is a simple fastener generally made of soft, malleable metal used to lock low torque parts in place. The split pin typically consists of a flat, round or half round pin bent double with a pronounced loop at the closed end. After a part has been put in place, the pin is passed through a hole in the shaft and one or both its “legs” bent back to prevent it working loose while the loop on the closed end stops it from falling through the hole. If the part has to be removed, the bent pin legs are simply straightened and the split pin removed.

Split pins are among the cheapest and most widely used locking fasteners for low speed, low stress parts. They may be found in a myriad of applications ranging from brake disc locks on heavy vehicles to paper puppet and plush toy joints. Also known as cotter pins, these fasteners are available in a range of sizes and designs including straight pins, spring pins, and hammer lock pins. There are also different cross section profiles available such as round, flat or half round. The split pin is also available with different end profiles including bevel ends, miter ends, and extended prong pins all of which are suited to specific uses.

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Part of the beauty of using split pins as fasteners is their sheer simplicity. Once the part to be locked is in place, the pin is pushed through an adjacent locking hole until the end loop or eyelet stops it going in any further. One or both legs are then bent back to stop the pin from working loose, thereby effectively preventing the part from exiting its housing or coming off the shaft. There are some split pin designs, such as the spring pin, which are a little more sophisticated though. These pins feature spring steel construction and a “belly” profile in one of the legs which locks around the outside surface of round shafts.

One of the well known uses of the split pin is the locking mechanism for the castellated nuts commonly used on automobile hubs. These lightweight nuts lock the hubs in place and exert an appropriate amount of torque on the wheel bearings. They feature a series of raised tabs along their upper edges reminiscent of castle battlements. The stub axle has a hole drilled through it, and once the nut is tightened to the correct torque, the split pin is inserted through the hole between the tabs on the nut. The ends are then bent backward to lock the pin and prevent the castellated nut from loosening.

Materials used in split pin manufacture tend to be soft, malleable metals such as aluminum, bronze, brass, and mild steel. This makes for easy bending of the legs when locking the pin in place. Unfortunately it also means the legs will only survive being bent once or twice before they break off. For this reason, split pins should only be used once and then discarded; this is quite acceptable when considering the low cost of these versatile little fasteners.

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